World's busiest airline flight routes in 2018: Melbourne - Sydney second busiest

So what the devil is Jeju, and why are so many rushing to see it?

It's actually the capital of an island, Jejudo, which might just be the most popular holiday destination you've never heard of.

Unesco-listed, and billed as South Korea's answer to Hawaii, it's pure Instagram gold, and home to dramatic volcanic landscapes, underground caves, hiking trails and scenic beaches.

Halla Mountain, at 1,940m above sea level, is South Korea's highest peak, while the cone of Seongsan Ilchulbong, or "Sunrise Peak", is particularly spectacular.

In 2011 Jejudo was named among the "New 7 Wonders of Nature", though there were accusations that its selection was down not just to its beauty, but also the readiness of tourism and marketing organisations to stump up cash – including taxpayers' money – to support its campaign.

There are also casinos, which help lure travellers from China, and, thanks to the island's self-governing status, anyone can visit without a visa.

Bizarrely, there's also a sex-themed park, Jeju Love Land, which features phallus statues, interactive exhibits on the "masterbation cycle", and other sculptures of humans in flagrante. Love Land is said to owe its existence to Jejudo's popularity as a honeymoon destination. Young newlyweds would arrive knowing next to nothing about the birds and the bees so some hotel employees offered to share their expertise. The island soon became an unofficial centre for sex education, making the theme park entirely logical.

But Jejudo also has a dark side. In 1948 and 1949, the South Korean government brutally put down an attempted uprising on the island. Villagers, including women and children, were massacred, and as recently as 2008 mass graves were still being uncovered. Some 30,000 people died as a result of the uprising, with a further 40,000 fleeing to Japan, and the atrocities are remembered at the 4.3 Peace Park memorial.

What do those crowded skies look like?

FlightRadar24 tracks every commercial aircraft on the planet and is a wonderful resource for aviation geeks like us. As the image above shows, there are planes flying across most corners of the planet at any given time. For jet-free skies, you'll need to head to Siberia, the northernmost reaches of Canada, the Amazon, the Sahara, the Australian Outback, or Madagascar.

Which is the world's busiest airport?

It's not quite as surprising as Jeju, but not far off. Nope, it's not Heathrow, Beijing or even Dubai. The busiest on the planet is actually Hartsfield-Jackson Atlanta International – as it has been since 1998 (20 years in a row). Almost 107.4 million passengers passing through its terminals in 2018 – that's slightly more than the population of the Philippines.

The biggest airline at Atlanta, by some margin, is Delta. It operates three-quarters of flights. And a remarkable 219 cities have non-stop services to it.

World's 10 busiest air routes:

1. Jeju-Seoul, South Korea: 79,460

2. Melbourne-Sydney, Australia: 54,102

3. Mumbai-Delhi, India: 45,188

4. Sao Paulo-Rio de Janeiro: 39,747

5. Fukuoka-Tokyo: 39,406

6. Hanoi-Ho Chi Minh City: 39,291

7. Sapporo-Tokyo: 39,271

8. Jakarta-Surabaya: 37,762

9. Los Angeles-San Francisco: 35,365

10. Jeddah-Riyadh: 35,149

World's busiest international air routes

1. Kuala Lumpur-Singapore: 30,187

2. Hong Kong-Taipei: 28,447

3. Jakarta-Singapore: 27,046

4. Hong Kong-Shanghai: 20,678

5. Jakarta-Kuala Lumpur: 19,741

6. Seoul-Osaka: 19,711

7. New York La Guardia-Toronto: 17,038

8. Hong Kong-Seoul Incheon: 15,770

9. Bangkok-Singapore: 14,698

10. Dubai-Kuwait: 14,581

The Telegraph, London

See also: World's safest, and least safe, airlines named in new rankings

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